Borehole logging is the method of making measurements and recording information about formations of the subsurface geology and/or the depth and angles of the borehole from the surface.
A log is recorded by visual inspection of rock samples from core samples extracted from the borehole and/or geophysical information obtained by one or more instruments lowered into the hole after the hole is drilled i.e. in an open hole.
Typically, the instrument recording the log is lowered into the hole using a winch. Logging is performed in boreholes drilled for mineral or oil and gas exploration or during groundwater geothermal, environmental or geotechnical surveying.
‘Wire-line’ logging is done in an open hole to acquire a complete picture of the rock properties of the subsurface formations. This information helps drilling operators and geologists to make decisions about drilling direction, presence/absence and direction of the resource being sought, or mining production.
A wire-line instrument is lowered down the hole to a chosen depth so that the petro-physical properties of formations can be captured and stored in a memory device for later analysis.
One or more sensors associated with the instrument can be activated to measure electrical, electromagnetic, natural gamma radiation levels or acoustic information to build a series of overlayed logs.
Traditional logging tools are connected via a cable connection that provides power and also provides a conduit to capture data in real time or raw data from the instrument is fed to a data acquisition system for later analyses.
Open hole logs are usually run before a hole is lined or cased off and they are managed by a costly technical team. Open hole survey costs are extremely high for the miner or explorer. Therefore, only a few holes can be logged, limiting the information available to a geologist when making drilling decisions.
‘Logging while drilling’ (LWD) technique has been pioneered in the oil and gas industry. LWD and also ‘measure while drilling’ (MWD) logs typically use mud pulse technology to transmit data from the downhole tool/instrument in the borehole for onward transmission to the surface for continuous analysis.
This technique provides the same information as wire-line logging, however, instead of lowering the instrument into the borehole on a winched cable, the required sensors are embedded in the drill string and the measurements are fed to the operator at the surface in real time to allow the drilling operators and geologist to obtain logs as well as information such as hole direction, weight on bit etc.
Data rates are slow (10 bits a second) and it is necessary to use data compression techniques and buffering to the tool's on board memory. Slow data transfer rates, complex technology to pass the data in any reliable way through mud pulses, and the need for a costly site technical team makes the use of such mud pulse technology undesirable. It has been realised that a more efficient data gathering methodology is desirable.
Geophysical methods can be used to interrogate a larger expanse of subsurface formations and can be used in conjunction with geological information from core samples to build a better geological prediction of the formations surrounding the core holes.
A geophysical measuring instrument, commonly referred to as a logging instrument or sonde, is lowered into the bore hole to collect relevant data. A sonde is essentially a ‘probe’ in the form, for example, of an electronic instrument arranged to sense one or more parameters or characteristics.
Logging instruments are already used in oil field operations to obtain information about the bore hole. This process is referred to as “logging”. Logging can be performed by “wireline logging”, “logging while drilling” (LWD) and “through-the-bit logging” after drilling has occurred.
In wireline logging, a logging instrument is lowered into the bore hole after the drill string has been extracted. The logging instrument hangs and is supported by a length of cable or “wireline”. Additionally, the wireline facilitates the electrical and communication connections between the logging instrument and the related equipment normally located at ground level.
In LWD the actual drilling assembly includes sensing instruments that measure required parameters as the bore hole is being drilled. Such sensing instruments are thus subject to the hostile downhole environment and consequently their operation is often compromised.
Through-the-bit logging involves introducing a logging instrument, for example a wireline tool, into the borehole through a central port in the drill bit located at the downhole end of the drill string. The logging instrument is lowered or pumped into the borehole through the interior passage of the drill string. The logging instrument is then passed through the port in the drill bit to enable logging of the borehole below the drill bit. Further, the instrument can be used to log the length of the borehole as the drill string is pulled out of the borehole. This process is often referred to as “logging while tripping”.
U.S. Pat. No. 8,443,915 describes different through-the-bit logging systems used for logging well bores drilled to extract crude oil and/or natural gas. As shown in FIGS. 1B and 1C of U.S. Pat. No. 8,443,915, a bottom hole assembly (BHA) includes a mill bit, a mud motor, a logging tool, a centralizer, a hanger and a disconnect. Once the well bore has been drilled to the required depth, the mill bit is used to cut through the nose of the drill bit to establish the port for the logging tool to pass through. The BHA is lowered through the bore of the drill string until the hanger is seated against an adapter of the drill bit.
Although logging instruments are used in oil and gas drilling operations, the use of such instruments has not readily translated to core sampling operations. This is due to the costs associated with providing specialist technicians and equipment at sampling sites to collect and record geophysical data.
The discussion of the background to the invention herein is included to explain the context of the invention. This is not to be taken as an admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of this application.
One or more forms of the present invention seeks to provide at least a method and/or logging instrument that enables geophysical data collection as part of a core sampling process, preferably utilising at least part of the drill rig equipment associated with core sampling.
One or more forms of the present invention has/have been developed with the aforementioned problems in the known art in mind.
It has been found desirable to develop geophysical logging apparatus and/or one or more methods of use and/or deployment thereof, or to develop a downhole logging method, that can be utilised or employed by drill operators/personnel as part of their normal operations without the need for additional specialist geophysical logging personnel.
Utilising the drill rig personnel/operators removes the need for specialist personnel and avoids interrupting drilling operations or avoids delays in otherwise waiting for specialist personnel to arrive on site after drilling. Logging operations can advantageously be carried out by the drill operators/personnel who require less training or specialisation.
Such an approach to drill logging operations is desirable in that it helps reduce the significant costs associated with exploration of subsurface formations, and is intended to also lower the barrier to adoption of the technology in a sector that is historically conservative to new technology and methods of working.